Current digital imaging devices for energetic particle detection, also called pixel detectors, can be classified into two broad classes, distinguished by the way in which impacting energy is converted into electrical signals. Taking X-ray photons as an example, in the first one of these classes the conversion happens indirectly in the sense that X-ray photons are first down-converted in energy to visible photons in a scintillation layer. The visible photons are subsequently detected by an array of photodiodes, in which the optical generation of electron-hole pairs gives rise to electrical signals which are then further processed by a readout electronics and represented as an image on a computer screen. The two-stage conversion process of indirect X-ray imaging devices suffers from the drawback of limited conversion efficiency and spatial resolution because of losses and scattering occurring both during the conversion of X-rays into visible photons and in the detection of those. Typically about 25 electron-hole pairs are finally measured by the readout electronics per keV of incident X-ray energy.
In the second class of these pixel detectors semiconductor absorbers permit the direct conversion of X-rays into electron-hole pairs which can then be measured as an electrical signal by a readout electronics. In addition to superior sensitivity and higher spatial and temporal resolution compared to scintillator based indirect conversion, such absorbers offer also spectral resolution, since the energy of an incident X-ray photon is proportional to the number of generated electron-hole pairs and thus measurable by a pulse height analysis. In silicon (Si), one needs on average 3.6 eV to create a single electron-hole pair (see for example R. C. Alig et al. in Phys. Rev. B 22, 5565 (1980); and R. C. Alig in Phys. Rev. B 27, 968 (1983), the entire disclosures of which are hereby incorporated by reference). On average this leads to 280 electron-hole pairs per keV of absorbed X-ray energy, from which it can be seen that the conversion efficiency exceeds that of a scintillator-photodiode combination by more than a factor of ten.
X-ray imaging detectors, or pixel sensors in general, employing direct conversion by means of semiconductor absorbers, can be implemented essentially in two different ways. In the first, an absorber wafer is bonded onto the readout chip in order to realize the connections needed to process the electrical signal from every absorber pixel. The most common bonding technique is bump bonding, as used for example by the Medipix collaboration (http://medipix.web.cern.ch) or by Dectris AG (http://www.dectris.ch). The absorber can in principle consist of any semiconductor material suitable for energetic particle detection from which large crystals can be grown, for example Si, Ge, GaAs and CdTe (see for example European Patent No. 0571135 to Collins et al., the entire disclosure of which is hereby incorporated by reference).
The second implementation of direct X-ray imaging detectors is based on the monolithic integration of the absorber with the readout electronics. Such monolithic pixel sensors with Si absorbers have been developed also for the detection of ionizing radiation other than X-rays in high energy physics. They comprise a high-resistivity absorber layer with a resistivity between about 1 kΩcm and 8 kΩcm epitaxially grown on the backside of a standard Si CMOS-wafer. The wafer is subsequently CMOS processed to fabricate the readout electronics on the front side (see for example S. Mattiazzo et al. in Nucl. Instrum. Meth. Phys. Res. A 718, 288 (2013), the entire disclosure of which is hereby incorporated by reference). While these devices are very promising for particle detection, absorbers with thicknesses much beyond those of epitaxial layers are needed for X-ray detection. Moreover, absorbers comprising elements with higher atomic number Z than Si (“heavier elements”) are more suitable for X-rays with energies above about 40 keV because of their more efficient absorption.
The monolithic integration of single crystal X-ray and particle absorbers from elements with higher Z in a monolithic unit with the Si readout electronics is, however, complicated by materials incompatibility, such as different lattice parameters and thermal expansion coefficients. Commercial devices are therefore based on polycrystalline or amorphous materials and readout circuits with thin film transistors. Such flat panel X-ray imaging detectors from amorphous selenium are already used for medical applications since they offer large size and are relatively inexpensive to make (see for example S. Kasap et al. in Sensors 11, 5112 (2011), the entire disclosure of which is hereby incorporated by reference). Since materials in the form of single crystals offer much better transport properties compared with their polycrystalline and amorphous counterparts, monolithic sensors made therefrom are, however, expected to offer much better performance. On the other hand, the practical realization of such structures has so far been hindered by the material incompatibility issues mentioned above.
There are a number of different ways in which a monolithic pixel sensor from single crystal high-Z materials can possibly be made. One approach is based on direct wafer bonding, wherein the absorber wafer is bonded onto the wafer containing the readout electronics. In practice the readout electronics comprises a CMOS-processed Si wafer. For example hydrophobic bonding may be used in order to assure an electrical connection between the bonded parts, which, however, requires special precautions to avoid hydrogen bubble formation during any low temperature annealing step, such as trench etching, which is ill-suited for detector applications (see for example U.S. Pat. No. 6,787,885 to Esser et al., the entire disclosure of which is hereby incorporated by reference).
In another approach the materials of readout wafer and absorber differ from Si but are essentially the same. It has for example been suggested to enrich Si with a heavier element such as Ge, giving rise to a SiGe alloy. An imaging and particle detection system based on bulk-grown Si1-xGex alloys with a Ge content below 20% has been disclosed in the International Patent Application No. WO 02/067271 to Ruzin, the entire disclosure of which is hereby incorporated by reference. In this proposed approach readout electronics and absorber are thus both fabricated in the same SiGe wafer. It requires, however, large SiGe wafers of sufficient quality to become available.
In yet another approach the absorber is epitaxially grown directly onto the CMOS-processed wafer containing the readout electronics. This has been disclosed for the example of an epitaxial Ge absorber in U.S. Pat. No. 8,237,126 to von Känel, the entire disclosure of which is hereby incorporated by reference. The large mismatch of lattice parameters of about 4.2% and thermal expansion coefficients of Ge and Si of about 130% near room temperature are highly problematic, however, since they result in high defect densities (such as misfit and threading dislocations and stacking faults), wafer bowing and layer cracks, all of which are serious obstacles in the way of producing an efficient device. Another difficulty with this approach is the limited temperature budget to which the CMOS readout circuits can be exposed. Typically, with standard aluminium metallization temperatures have to be kept below 450° C. This is too low for high-quality Ge epitaxy to be maintained to the layer thicknesses of dozens of micrometers needed for efficient absorption of high-energy X-ray photons. The only way to deposit thick Ge-layers in a backend process appears hence to exist through the use of a modified, temperature resistant metallization, for example a tungsten metallization as offered by some companies.
For reasons of manufacturing costs, scaling to large area absorber wafers, suitable for example for the fabrication of flat panel detectors, is highly desirable irrespective of the design details of a pixel detector. As Si wafers of excellent quality are readily available with sizes of 300 mm and beyond, the use of thick epitaxial layers of high-Z materials on Si substrates appears to be an attractive alternative to bulk crystal growth. The epitaxial growth of most compound semiconductors is, however, even more difficult than that of Ge since in addition to the lattice and thermal mismatch one faces the problem of anti-phase domain formation because of different step heights of substrate and epilayer. For what concerns the application in X-ray imaging detectors these problems have been largely ignored (see for example European Patent Application No. 1 691 422 to Yasuda, the entire disclosure of which is hereby incorporated by reference).
The problem of wafer bowing and layer cracking has been solved by a method involving deep Si-substrate patterning at a micron-scale, along with far-from-equilibrium epitaxial growth, giving rise for example to space-filling Ge-crystals separated by tiny gaps (see for example International Patent Application No. WO 2011/135432 to von Känel, the entire disclosure of which is hereby incorporated by reference). For sufficiently large aspect ratio of the crystals for faceted surfaces, the method leads furthermore to the expulsion of all threading dislocations, so that crystal regions at a distance of several microns from the interface are entirely defect-free (see for example C. V. Falub et al. in Sci. Rpts. 3, 2276 (2013), the entire disclosure of which is hereby incorporated by reference). In a modification of the detector concept of U.S. Pat. No. 8,237,126 to von Känel (see Kreiliger, Physica Status Solidi A 211, 131-135 (2014), the entire disclosure of which is hereby incorporated by reference) the Ge-absorber, consisting of isolated, densely spaced Ge-crystals, is located on the back-side of a Si-wafer on the front-side of which the readout electronics has been incorporated by CMOS-processing. The electron-hole pairs generated within the Ge-absorber therefore need to be separated and, depending on polarity, electrons or holes have to cross the depleted Si/Ge heterojunction (the Si wafer and Ge absorber form a heterojunction diode) and drift through the Si-wafer in order to be collected by implants on the readout side the spacing of which defining the pixel size. The concept has two major drawbacks: (1) the Si/Ge interface necessarily harbours a very high density of misfit dislocations because of the lattice mismatch of 4%. These dislocations act as generation/recombination centers, forming an important contribution to the reverse current of the Si—Ge diode in the dark (see for example Colace et al. in IEEE Photonics Technology Letters 19, 1813 (2007), the entire disclosure of which is hereby incorporated by reference); (2) Pure Ge is not an ideal material for applications in large area detectors because of its low room temperature resistivity of only 50 Ωcm. For this reason Ge detectors have to be cooled typically to liquid nitrogen temperature (see for example U.S. Pat. No. 5,712,484 to Harada and http://www.canberra.com/products/detectors/germanium-detectors.asp, the entire disclosures of which is hereby incorporated by reference).
It is the aim of the invention to provide monolithic pixel sensors based on CMOS processed readout electronics and both lattice and thermally matched as well as mismatched absorber layers without the need of any special high-temperature metallization layers. The combination of readout electronics wafer and absorber wafer in a monolithic unit is provided by an electrically conductive bond obtained by room-temperature covalent bonding. Strong covalent wafer bonding carried out near room temperature are possible for example by means of equipment manufactured by EV Group (see for example C. Flötgen et al. in ECS Transactions 64, 103 (2014), the entire disclosure of which is hereby incorporated by reference). The invention is equally applicable to monolithic pixel detectors with Si absorbers and absorbers made from high-Z materials. In particular, it provides large area monolithic pixel sensors, for example for use in flat panel detectors, even for high-Z absorber materials for which at present no large wafers can be manufactured at a bearable cost. Depending on the application, it is based on the covalent bonding of a thinned Si wafer containing the readout electronics either with a thinned Si wafer acting as absorber, a thinned Si wafer carrying an epitaxial absorber layer, or a thick absorber wafer made from any semiconductor material of high quality.